What Does the T Temper Designation Mean?
The T prefix in an aluminum temper identifies a heat-treatable condition. For 6061, the tempering route begins with solution heat treatment at about 530 degrees C, which dissolves the magnesium and silicon into solid solution, followed by quenching to freeze that structure. Artificial aging at around 160 degrees C then precipitates fine strengthening phases, converting the alloy to the T6 condition. The digits after the T describe additional processing: 6 means artificially aged to peak strength, 51 means stress relieved by stretching, and a final 1 means straightened by mechanical means. Understanding these steps explains why three tempers with identical strength can perform very differently in machining and forming.
6061-T6: The Baseline Temper
6061-T6 is the most common 6061 temper for sheet, plate and general products. After solution heat treatment and artificial aging, it reaches minimum tensile strength of about 290 MPa and yield strength of about 240 MPa. Because no stress relief is applied, the material retains internal stresses from quenching. In practice this means T6 plate can distort or warp during heavy machining, when the removal of material releases locked-in stress. T6 is also the most ductile of the three tempers, which makes it preferable where the part will be bent or formed after heat treatment, and it is the least expensive because the production route is shortest.
6061-T651: Stress Relieved by Stretching
6061-T651 follows the same heat treatment as T6 and then adds a controlled stretching step, typically a permanent set of 1-3%, which relieves most of the internal stress. The result is a material with the same strength as T6 but far greater dimensional stability during machining. T651 is the standard choice for precision-machined parts, tooling plates, molds and any component where flatness and stability after metal removal are critical. Plate in T651 is widely stocked and is the recommended temper when a machinist will remove significant material from one side. The stretching operation may leave faint marks on the surface, but these do not affect performance.
6061-T6511: Stretched and Straightened
6061-T6511 is produced by stretching for stress relief and then mechanically straightening the product, which removes nearly all residual stress and ensures excellent dimensional accuracy. This temper is typical for extruded products such as rods, bars, tubes and profiles, where straightness along the length is essential. T6511 offers the best dimensional stability of the three tempers, making it the first choice for long extruded sections that will be machined or used with tight alignment requirements. It is also the most expensive, since two additional processing steps are involved, and its ductility is slightly lower than T6 because of the extra working.
Comparing the Three Tempers
In terms of strength, the three tempers are equivalent, with the same typical tensile and yield values, so the decision is not about strength but about stability, formability and cost. For stability during machining, T651 beats T6 and T6511 beats T651. For ductility and forming, T6 is the best, followed by T651 and then T6511. For availability, T6 is common in sheet and plate, T651 in rolled plate, and T6511 in extruded rod, bar and profile. For cost, T6 is lowest, T651 moderate and T6511 highest. Applications follow the same logic: bicycle frames and formed parts use T6, tooling plates and precision components use T651, and extruded structural bars and tubes use T6511.
FAQ
Is 6061-T651 stronger than 6061-T6?
No. T651 and T6 have the same minimum tensile and yield strength; T651 simply adds stress relief by stretching. The practical difference is dimensional stability during machining, not strength.
Why does 6061-T6 warp during machining?
Quenching during heat treatment creates internal stresses in T6 material. When machining removes material unevenly, those locked-in stresses redistribute and the part distorts. Stress-relieved tempers such as T651 and T6511 are designed to avoid this problem.
Which temper is best for precision-machined parts?
For plate and sheet parts, T651 is the standard choice because it stays flat during machining. For extruded bars and profiles, T6511 provides the best straightness and stability. Both are worth the small cost premium over T6 when tolerances are tight.
Can I bend 6061-T651 plate?
Yes, but like T6 it has limited ductility compared with the O temper. Use generous bend radii, bend parallel to the rolling direction where possible, and for severe forming consider annealing to O, forming, and re-heat-treating if the final strength is required.
Does T6511 affect the surface finish?
The stretching and straightening operations can leave light tooling or die marks on the surface of T6511 products. These are cosmetic and do not affect mechanical performance. If a pristine surface is required, specify machined or polished finishes.